Plasma arc torches are widely used in the cutting, welding and heat treating of metallic materials. A plasma arc torch generally includes a cathode block with an electrode mounted therein, a nozzle with a central exit orifice mounted within a torch body, a shield, electrical connections, passages for cooling and arc control fluids, a swirl ring to control fluid flow patterns in the plasma chamber formed between the electrode and nozzle, and a power supply. The torch produces a plasma arc, which includes a constricted ionized jet of a conductive plasma gas with high temperature and high momentum. The plasma gas, when energized by a DC source, forms a current path between the electrode and the nozzle (positive potential) creating the plasma arc pilot. Placing the nozzle near the workpiece causes the current path to flow between the workpiece and the electrode because the workpiece rests at a higher positive potential then the nozzle. Many of the torch components are consumable in that they deteriorate over time and require replacement. These “consumables” include the electrode, swirl ring, nozzle, retaining cap, and shield.
Frequently during torch operation, the operator is constrained by space or visibility, which may lead to inadvertent contact of the side of the nozzle to the workpiece resulting in “double arcing.” Double arcing is a condition where the plasma arc deviates from its intended electrode to workpiece path and instead goes from the electrode to the nozzle and then to the workpiece—causing electrical continuity between the nozzle and the workpiece. Double arcing causes premature wear to the nozzle and results in frequent nozzle replacement and additional expense. In addition, double arcing can cause nozzle stickiness, which inhibits accurate hand control of the torch. The use of a shield, which is electrically floating, around the nozzle helps to eliminate the risk of double arcing, but currently available shields have undesirable limitations.
Despite nozzle shields being pervasive in the commercial market, they are often bulky and inhibit visibility of the plasma arc by the operator. One design difficulty for conductive shields is establishing a sufficient dielectric gap. That is, a conductive shield must be positioned or spaced away from the nozzle to prevent the plasma arc from jumping from the nozzle to the shield. The desired gap or distance between the shield and nozzle is a function of the dielectric strength of the medium within the gap, gas dynamics, metal contamination within the gap, tolerance stack up, and the physical condition of the shield and/or nozzle. The arcing distance is the minimum distance required between a conductive shield and a nozzle to prevent the plasma arc from jumping the gap between the shield and the nozzle. In conventional torches, the conductive shield is positioned at least an arcing distance away from the nozzle causing the total covered volume surrounding the plasma arc to be large, thereby reducing operator visibility.
A ceramic shield can be used in place of a conductive shield, but problems associated with these consumables exist. One difficulty with ceramic shields in plasma arc torch systems, despite their ability to solve the spacing and electrical isolation problems, is that they cannot withstand the thermal and impact shocks that occur during normal industrial use. In addition, ceramic shields are generally bulky and therefore decrease operator visibility. Moreover, ceramic shields are often too brittle for most hand torch systems.